[Python-Dev] PEP 549: Instance Properties (aka: module properties)

Larry Hastings larry at hastings.org
Sun Sep 10 23:52:12 EDT 2017

On 09/06/2017 02:13 PM, Ronald Oussoren wrote:
> To be honest this sounds like a fairly crude hack. Updating the 
> __class__ of a module object feels dirty, but at least you get normal 
> behavior w.r.t. properties.

Okay.  Obviously I disagree, I think it's reasonable.  But I'll assume 
you're -1.

> Why is there no mechanism to add new descriptors that can work in this 
> context?

I've updated the prototype to add one.  I added it as 
"collections.abc.InstanceDescriptor"; that's a base class you can 
inherit from, and then your descriptor will work in a module. 
Bikeshedding the name is fine.

> BTW. The interaction with import is interesting… Module properties 
> only work as naive users expect when accessing them as attributes of 
> the module object, in particular importing the name using “from module 
> import prop” would only call the property getter once and that may not 
> be the intended behavior.

I spent a fair amount of time thinking about this.  The short answer is: 
we /could/ fix it.  We /could/ make it so that "from x import y", when 
x.y is an instance descriptor, ensures that y honors the descriptor 
protocol when referenced.  We'd have to do it for three contexts:

  * global scope (aka module scope)
  * class scope
  * function scope

The first two are pretty similar; create a proxy object that retains the 
module instance so it remains "bound" to that.  The third one is 
trickier; it'd mean a new bytecode (LOAD_FAST_DESCRIPTOR), but it 
wouldn't slow down people who didn't use it.

Anyway, long story short, I think this would be worse than simply having 
"from x import y" only calling the getter once.  As the Zen says: 
special cases aren't special enough to break the rules.

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